Detailed Exergy Analysis Research Articles

Modelling of advanced gasification systems (MAGSY): Simulation and validation for the case of the rising co-current reactor

This study presented a new model (named MAGSY) for the simulation of advanced small-scale gasification systems. These novel designs can be complex and may combine several features that are not typical for conventional systems. MAGSY is a vis-à-vis and process-based model that has been able to account for the excess air in the combustion sub-process (zone) which is conveyed to the pyrolysis zone creating practically a coupled system of sub-stoichiometric combustions. Onsite monitoring results from a rising co-current gasifier are used for the validation of the model. Simulations have been performed for excess air regimes between 11% and 20% and the produced gases from the “oxidation reactions” system were reformed by the “char-gas reactions” system. The model calculated a hydrogen molar fraction of 19.53% and carbon monoxide molar fraction of carbon monoxide 24.45% while the case study values were 19.19% and 24.02% respectively. Both gases were shown to have inverted correlation with equivalent air ratios at values bigger than 99.3%, which is a notable point. For equivalent ratios of 0.l7–0.25 the model returned char yields of 1.2–1.7% and the measurement from the case study showed that the char yield was 1.5%. Finally, a detailed exergy analysis of the gasifier is presented and validated with the values that were retrieved from the monitoring. MAGSY is able to adjust and model reactors with different levels of complexity.

Energy and exergy analysis of a modified absorption cycle: A comparative study

Detailed exergy analysis is conducted on a branched generator-absorber heat exchanger (branched GAX) cycle and the system performance is compared to that for the GAX cycle thermodynamically. By considering the physical and chemical exergies, exergy destruction in each component of the system is calculated and its contribution to the overall exergy destruction is revealed. Parametric studies are also performed to investigate the effects on the coefficient of performance (COP) and the exergy efficiency of some design variables. The results indicate better performance for the branched GAX cycle from the viewpoints of energy and exergy efficiencies. The branched GAX system, however, depends strongly on the condenser temperature which can be accounted as a disadvantage. It is observed that the branched GAX gains an advantage of about 14% in the COP and exergy efficiency for the condenser temperature of 30 °C, however, this advantage goes away as the condenser temperature reaches 50 °C. Furthermore, the total exergy destruction in the branched GAX cycle amounts to 5.17 kW, i.e., 10.6% lower than that for the GAX cycle.

Exergetic analysis of an LPG production plant using HYSYS software

This work is a contribution on thermo-economic analysis of a Liquefied Petroleum Gas (LPG) unit production which is simulated in Aspen HYSYS. The fractional unit of the plant will be studied in order to locate exergy destruction area in the system. The fractional unit contains mainly three sections: heat recovery, rectification columns and cooling section. Also we will determine the exergy for all streams, which will be useful for the thermo-economic analysis. As a result, two thirds of exergy destruction is in deethanizer, depropanizer, debutanizer composing the rectification section. So a detailed exergy analysis is developed for each section.

Energy and exergy analyses of a HCCI engine-based system running on hydrogen enriched wet-ethanol fuel

In this paper, the investigation of thermodynamic performance of a hydrogen enriched wet-ethanol fuelled homogenous charge compression ignition (HCCI) engine is conducted through energy and exergy methods. The energy and exergy analyses are performed through a thermodynamic model by considering the gas composition formed after the combustion process. The energy and exergy efficiencies of the system increase from 41.56% to 44.01% and from 37.23% to 38.73%, respectively, with the increase in hydrogen energy share from 5% to 20% in the air-fuel mixture. The energy and exergy efficiencies of the system are found to be increasing with an increase in the pressure ratio of turbocharger and compressor polytropic efficiency. The increase in the ambient temperature results in the decrease in both energy and exergy efficiencies. The detailed exergy analysis of the system reveals that the highest exergy destruction is associated with HCCI engine, and the maximum available work increases with the increase in the hydrogen percentage.

Energy and exergy analyses of a HCCI engine-based system running on hydrogen enriched wet-ethanol fuel

In this paper, the investigation of thermodynamic performance of a hydrogen enriched wet-ethanol fuelled homogenous charge compression ignition (HCCI) engine is conducted through energy and exergy methods. The energy and exergy analyses are performed through a thermodynamic model by considering the gas composition formed after the combustion process. The energy and exergy efficiencies of the system increase from 41.56% to 44.01% and from 37.23% to 38.73%, respectively, with the increase in hydrogen energy share from 5% to 20% in the air-fuel mixture. The energy and exergy efficiencies of the system are found to be increasing with an increase in the pressure ratio of turbocharger and compressor polytropic efficiency. The increase in the ambient temperature results in the decrease in both energy and exergy efficiencies. The detailed exergy analysis of the system reveals that the highest exergy destruction is associated with HCCI engine, and the maximum available work increases with the increase in the hydrogen percentage.

Design, Construction, and Testing of a Gasifier-Specific Solid Oxide Fuel Cell System

This paper describes the steps involved in the design, construction, and testing of a gasifier-specific solid oxide fuel cell (SOFC) system. The design choices are based on reported thermodynamic simulation results for the entire gasifier- gas cleanup-SOFC system. The constructed SOFC system is tested and the measured parameters are compared with those given by a system simulation. Furthermore, a detailed exergy analysis is performed to determine the components responsible for poor efficiency. It is concluded that the SOFC system demonstrates reasonable agreement with the simulated results. Furthermore, based on the exergy results, the components causing major irreversible performance losses are identified.

Novel process for the exergetically efficient recycling of chlorine by gas phase electrolysis of hydrogen chloride

Due to the steadily increasing production capacities of polycarbonates and polyurethanes, the volume of hydrogen chloride (HCl), which emerges as byproduct from these processes, is experiencing a significant growth. Owing to the oversaturation of the market for HCl and hydrochloric acid, the question on how to utilize this byproduct in a sustainable and energy-efficient way is becoming more and more important. The oxidation of HCl to chlorine, which can be redirected as an educt to these above mentioned processes, offers a feasible solution to this problem and can be carried out either heterogeneously catalyzed at high temperatures or through electrolysis. Up to now, the most energy-efficient industrially employed electrochemical variant is the Bayer UHDENORA process, based on aqueous hydrochloric acid as a feed stock. The major objective of this work is to propose a novel electrochemical process utilizing HCl as a gaseous reactor feed in order to significantly reduce the electrical energy demand of the reactor combined with new and more energy-efficient subsequent separation sequences. For this purpose, flowsheet simulations of the Bayer UHDENORA process and two novel process variants based on the gas phase reactor and two different separation sequences were carried out and followed by a detailed exergy analysis. The analysis shows significant exergetic savings of up to 38% in this novel process variants compared to the Bayer UHDENORA process, including not only the electrochemical reactor but also the subsequent separation steps. The novel process therefore allows for a more sustainable and energy-efficient production of the base chemical chlorine and, contrary to the heterogeneously catalyzed high temperature process, for the utilization of electrical energy from advancing renewable sources like wind and sun.

Sustainability assessment of turbofan engine with mixed exhaust through exergetic approach

In this study, the theory, methods and example application are described for a CF6 high-bypass turbofan engine with mixed exhaust flow based on exergo-sustainable point of view. To determine exergetic sustainability index, the turbofan engine has to undergo detailed exergy analysis. The sustainability indicators reviewed here are the overall exergy efficiency of the system, waste exergy ratio, exergy destruction factor, environmental effect factor and the exergetic sustainability index. The results obtained for these parameters are 26.9%, 73.1%, 38.6%, 2.72 and 0.37, respectively, for the maximum take-off condition of the engine. These results would be useful to better understand the connection between the propulsion system parameters and their impact to the environment in order to make it more sustainable for future development.

Exergetic analysis of deep-bed drying of rough rice in a convective dryer

This research work was aimed at a detailed exergy analysis of paddy deep-bed drying process in a laboratory-scale convective dryer. The drying experiments were conducted at different inlet drying air temperatures and flow rates, and exergy loss, exergy destruction, exergy efficiency, improvement potential rate and sustainability index were investigated. Normalized exergy destruction of the process ranged from 1.67 to 7.46. Exergy efficiency of the drying process and drying chamber was specified to be in the ranges of 5.10–29.41% and 32.64–67.75%, respectively. The results indicated that applying higher drying air temperatures resulted in higher exergy efficiencies. The obtained improvement potential rates showed that the process is highly capable of ameliorating exergy performance improvement. The sustainability index of drying process ranged from 1.05 to 1.42, augmenting with the increase in the air temperature.

Integration of exergy analysis into model-based design and evaluation of aircraft environmental control systems

The environmental control system of an aircraft is a complex energy intensive system and the most important non-propellant consumer of energy among all aircraft systems. Considering the highly competitive market for conventional aircraft, meaningful analysis methods for evaluation during conceptual design of ECS and effective modelling and simulation tools became more important. Hence, this paper focuses on energy and exergy analyses applied to the conventional aircraft environmental control system combined with the model-based design approach. The reported results are related to the different analysis methods for aircraft environmental control systems and cover the model-based design approach.The exergy method is applied to a model of a conventional air generation unit of a commercial aircraft. This model was developed using a model-based design framework. Within this framework, the exergy analysis was performed for four phases of a standard flight mission: Take-off, cruise, landing and taxi. The results obtained from the detailed exergy analysis show highly varying performances for the components at the different simulation cases. The exergy efficiencies range from 1% to 88%. A discussion about the special requirements for aircraft environmental control system analysis methods concludes the paper.

Thermodynamic analysis of low-grade solar heat source-powered modified organic Rankine cycle using zeotropic mixture (Butane/R1234yf)

ABSTRACTThis paper communicates detailed energy and exergy analysis of low-grade energy resource of solar-powered organic Rankine cycle (ORC) integrated with both the internal heat exchanger and open feed water heater, ORC incorporated with the internal heat exchanger, with open feed water heater and basic ORC, respectively. Results indicate that highest first law efficiency (11.9%), exergetic efficiency (51.88%) and lowest exergy destruction (1749 kW) are obtained for ORC integrated with both internal heat exchanger and open feed water heater among other considered ORCs. Moreover, zeotropic mixture (butane/R1234yf) shows better first law and exergetic efficiency and lower exergy destruction than pure fluid.

Comparative study of two weir type cascade solar stills with and without PCM storage using energy and exergy analysis

In this paper, the comparative study of energy and exergy performance of two weir type cascade solar stills with and without PCM storage in sunny and semi-cloudy days is carried out. The governing equations of energy analysis include a set of nonlinear equations which is obtained by writing energy balance for the various components of a solar still (i.e. glass cover, brackish water, absorber plate, phase change materials). A detailed exergy analysis is carried out and various irreversibility rates in the solar still system and its exergy efficiency are introduced. In order to solve the governing equations a computer simulation program is developed. The results of a numerical simulation of the present study are in good agreement with the experimental data of previous literatures. The numerical results of the present study show that the energy and exergy performance of solar still without PCM storage is better than the solar still with PCM storage in sunny days. On the other hand, the solar still with PCM storage is preferred for semi-cloudy days due to its better energy and exergy performance. The maximum value of the energy and exergy efficiencies of the solar still without PCM for a typical sunny day are 76.69% and 6.53%, respectively. While, the maximum energy and exergy efficiencies of the solar still with PCM for a sample semi-cloudy day are 74.35% and 8.59%, respectively. Furthermore, it is observed that the highest irreversibility rate belongs to the absorber plate and its value for the solar still without PCM on typical sunny day and the solar still with PCM on semi-cloudy days is 83.1% and 78.8% of the whole of system irreversibility rates, respectively. Whereas, the irreversibility rate of glass cover and brackish water can be neglected.

Exergy analysis of a solar-powered vacuum membrane distillation unit using two models

A detailed exergy analysis of a solar powered VMD unit was performed using two models: the ideal mixture model and the model using the thermodynamics properties of seawater. The exergy flow rates of process steam, given by the two models differed of about 18%, on average. Despite these differences, the two models agree that during the step of condensation, the most important fraction of exergy was destroyed. Moreover, in this work, two forms of exergy efficiency are calculated. The overall exergy efficiency of the unit with reference to the exergy collected by the solar collector was 3.25% and 2.30% according to Cerci and Sharqawy models, respectively. But, it was 0.182% and 0.128%, when referenced to the exergy of solar radiation, according to Cerci and Sharqawy models, respectively. Besides, the utilitarian exergy efficiency was 9.96%. Since the heat exchanger, the hollow-fiber module and the condenser have a very high exergy performance, then it can be concluded that the enhancement or reduction of exergy losses will be mainly by recovering heat lost in brine discharges and in the rejection of the cooling water. In addition, the influence of the rejection rate on exergy efficiencies was studied.

Towards retrofitting integrated gasification combined cycle (IGCC) power plants with solid oxide fuel cells (SOFC) and CO2 capture – A thermodynamic case study

This article presents a detailed thermodynamic case study based on the Willem-Alexander Centrale (WAC) power plant in the Netherlands towards retrofitting SOFCs in existing IGCC power plants with a focus on near future implementation. Two systems with high percentage (up to 70%) biomass co-gasification (based on previously validated steady state models) are discussed: (I) a SOFC retrofitted IGCC system with partial oxy-fuel combustion CO2 capture (II) a redesigned highly efficient integrated gasification fuel cell (IGFC) system with full oxy-fuel CO2 capture. It is concluded that existing IGCC power plants could be operated without major plant modifications and relatively high electrical efficiencies of more than 40% (LHV) by retrofitting SOFCs and partial oxy-combustion CO2 capture. In order to apply full scale CO2 capture, major process modification and redesign needs to be carried out, particularly in the gas turbine unit and heat recovery steam generator (HRSG). A detailed exergy analysis has also been presented for both the systems indicating significant efficiency improvement with the utilization of SOFCs. Additional discussions have also been presented on carbon deposition in SOFCs and biomass CO2 neutrality. It is suggested that scaling up of the SOFC stack module be carried out gradually, synchronous with latest technology development. The thermodynamic analysis and results presented in this article are also helpful to further evaluate design challenges in retrofitted IGCC power plant systems for near future implementation, gas turbine part load behaviour, to devise appropriate engineering solutions and for techno-economic evaluations.

Exergy and exergoeconomic analyses of a supercritical CO2 cycle for a cogeneration application

Detailed exergy and exergoeconomic analyses are performed for a combined cogeneration cycle in which the waste heat from a recompression supercritical CO2 Brayton cycle (sCO2) is recovered by a transcritical CO2 cycle (tCO2) for generating electricity. Thermodynamic and exergoeconomic models are developed on the basis of mass and energy conservations, exergy balance and exergy cost equations. Parametric investigations are then conducted to evaluate the influence of key decision variables on the sCO2/tCO2 performance. Finally, the combined cycle is optimized from the viewpoint of exergoeconomics. It is found that, combining the sCO2 with a tCO2 cycle not only enhances the energy and exergy efficiencies of the sCO2, but also improves the cycle exergoeconomic performance. The results show that the most exergy destruction rate takes place in the reactor, and the components of the tCO2 bottoming cycle have less exergy destruction. When the optimization is conducted based on the exergoeconomics, the overall exergoeconomic factor, the total cost rate and the exergy destruction cost rate are 53.52%, 11243.15 $/h and 5225.17 $/h, respectively. The optimization study reveals that an increase in reactor outlet temperature leads to a decrease in total cost rate and total exergy destruction cost rate of the system.

Application of exergy analysis to the dairy industry: A case study of yogurt drink production plant

This work was aimed at undertaking a detailed exergy analysis of an industrial-scale yogurt drink production plant using actual operational data. Exergy efficiency and exergy destruction rate of each subcomponent of four main subsystems of the plant, including steam generation, above-zero refrigeration, milk reception, pasteurization, and standardization, and yogurt drink production lines were derived independently. This analysis was performed to quantify thermodynamic inefficiencies of all subcomponents of the plant in order to identify the breakthrough points for further energy savings. An effort was also made to compute the quantity of exergy destruction in processing a given amount of pasteurized yogurt drink. The results indicated that the highest exergy destruction rate occurred in the boiler & compressor combination of steam generator, followed by ice-water tank & agitator combination in the above zero refrigeration system. The specific exergy destruction of pasteurized yogurt drink was determined as 442kJ/kg according to the mass allocation method. The steam generator had the highest contribution to the specific exergy destruction of pasteurized yogurt drink, followed by yogurt drink production, above-zero refrigeration, and milk reception, pasteurization, and standardization lines, respectively. In general, the current survey can be an important step for future improvements of dairy processing plants from the sustainability and productivity viewpoints. This study clearly showed the effectiveness of exergy analysis for determining irreversibilities and losses occurring in dairy processing plants in order to improve their thermodynamic performances.

System design and exergetic evaluation of a flexible integrated reforming combined cycle (IRCC) power plant system with carbon dioxide (CO2) capture and metal hydride based hydrogen storage

Natural gas combined cycle (NGCC) power plants with pre-combustion CO2 capture called, integrated reforming combined cycle (IRCC) power plants could be operated in a flexible and environment friendly manner by incorporating hydrogen storage. The current article presents a thermodynamic model evaluation of such a flexible large scale power plant system with co-production of hydrogen and subsequent hydrogen storage with metal hydrides (MH). A comparative analysis has been presented using steady state ASPEN Plus models for a Reference NGCC system, an IRCC system and a Flexible IRCC system with MH (MgH2) based hydrogen storage. In addition, a detailed exergy analysis indicating irreversibilities in the system has been presented. The system study indicates that such Flexible IRCC systems could be operated with a time based average (considering charging and discharging) electrical efficiency above 45% depending on the amount of hydrogen stored, appropriate heat integration and choice of the metal hydride. Exergy analysis indicates that such flexible systems could still be operated at a relatively high average net exergy efficiency (considering charging and discharging). Addition of MH based H2 storage in an IRCC system does not lead to significant electrical/exergy efficiency penalty. Carbon capture and storage is a larger source of efficiency penalty than the MH based H2 storage unit. The largest exergy destruction is observed in the gas turbine combustor and reformer units, providing additional scope for efficiency enhancement. With a larger share of renewable power in the total electrical power production expected in future, Flexible IRCC systems as presented in this article could be used as an efficient solution to address the large fluctuations in power supply and demand. Additional investigations are however encouraged further toward optimizing the choice of the metal hydride, detailed design of a single heat integrated system (including part load behavior) and economic assessments based on the heat and mass balances developed in this study.

Exergy Analysis of Data Centres-Effect of the Rack Location and Airflow Direction on the Thermal Performance

In this paper, a numerical analysis of flow and temperature distribution of a small raised-floor data centre is conducted in order to evaluate the thermal performance of the data centre. The flow patterns and temperature profiles established leads to a detailed exergy analysis of the data centre, which results in better understanding of irreversibilities in room airspace. The impact of the rack location in the data centre room and the airflow direction through perforated tiles on the thermal performance of the data centre is investigated using first-law and second-law of thermodynamics.

Exergy and exergoeconomic assessments of an electricity production system in a running wastewater treatment plant

This study aims at presenting a detailed exergy and exergoeconomic analyses of a biogas engine powered electricity production system in a wastewater treatment plant located in Adana, Turkey. The overall system consists of a compressor, a turbine, a pump, four heat exchangers and a gas engine. The effect of the each system component on the exergy efficiency and exergoeconomic parameters is analyzed separately. The analysis indicates that major exergy destruction happens in the gas engine as 4055.31 kW, followed by the exhaust gas heat exchanger (EGHE) and lubrication oil heat exchanger (LOHE) as 99.86 kW and 92.64 kW, respectively. The second law efficiency of the compressor, turbine, pump, BGHE, LOHE, WHE, EGHE and gas engine are found as 76.50%, 78.43%, 6.49%, 13.40%, 11.56%, 60.44%, 67.36%, and 50.79%, respectively. The operating and maintenance costs, capital investment cost and total cost of the system are reckoned as 14.74 US$/h, 13.90 US$/h, and 28.64 US$/h, respectively to point out the exergoeconomic performance. The unit exergy cost and exergy cost of the fuel, which feeds the compressor, are reckoned as 4.88 US$/GJ and 141.12 US$/h. The exergy cost and electricity production unit exergy cost in the turbine are found as 4.87 US$/h and 11.32 US$/GJ.

Exergy analysis of an industrial-scale ultrafiltrated (UF) cheese production plant: a detailed survey

In this study, a detailed exergy analysis of an industrial-scale ultrafiltrated (UF) cheese production plant was conducted based on actual operational data in order to provide more comprehensive insights into the performance of the whole plant and its main subcomponents. The plant included four main subsystems, i.e., steam generator (I), above-zero refrigeration system (II), Bactocatch-assisted pasteurization line (III), and UF cheese production line (IV). In addition, this analysis was aimed at quantifying the exergy destroyed in processing a known quantity of the UF cheese using the mass allocation method. The specific exergy destruction of the UF cheese production was determined at 2330.42 kJ/kg. The contributions of the subsystems I, II, III, and IV to the specific exergy destruction of the UF cheese production were computed as 1337.67, 386.18, 283.05, and 323.51 kJ/kg, respectively. Additionally, it was observed through the analysis that the steam generation system had the largest contribution to the thermodynamic inefficiency of the UF cheese production, accounting for 57.40 % of the specific exergy destruction. Generally, the outcomes of this survey further manifested the benefits of applying exergy analysis for design, analysis, and optimization of industrial-scale dairy processing plants to achieve the most cost-effective and environmentally-benign production strategies.